1. Field of the Invention
The present invention relates to X-ray detectors and, more particularly, to the technical field of X-ray detectors utilizing selenium (Se).
2. Description of the Related Art
X-ray detectors utilizing radioactive rays such as X-rays and X-rays are recently playing a role of increasing importance in a wide range of medical and industrial applications. In the medical field of application, X-ray CT is a typical example of the use of digital images in diagnosis. Transition from analog photography to digital photography is represented by DSA (Digital Signature Algorithm), and CR(DR)s for general simple photography for which a great amount of demand exists. Digital ultrasonic apparatuses have recently been actively developed, too. The digitalization in almost all fields of imaging technology is supported by the fact that image quality of analog photography has now become available on a digital basis, reductions in the prices of photographic apparatuses and various advantages provided by the use of image preservation and transfer systems.
Above all, there is a great demand in the market for methods for recording and reading X-ray images because of many advantages expected from such methods including utilization of digitized image information for general purposes, improvement of diagnostic efficiency and quality, and cost reduction achieved by connection of an image network.
X-ray images are recorded and detected using silver-salt films, organic polymer films or accelerated phosphorescence films and semiconductor devices. However, the above-described market demand for digitalization is still unsatisfied, for example, in that an accelerated phosphorescent element does not allow real time image processing because of a time lag in image processing attributable to the fact that the element utilizes the phenomenon of temporarily accumulating X-ray energy and emitting fluorescent light when irradiated by visible light.
In order to solve such a problem, development efforts have been put on image processing systems utilizing X-ray image sensors represented by direct conversion type devices on TFT (thin film transistor) panels, image processing systems utilizing an indirect type image reading sensor comprising a scanning laser and an LED, and the like. The development is also active on utilization of unique characteristics of amorphous Se which is a light-receiving element for X-ray image sensors.
A common technique for forming Se films is vacuum deposition which has been established as a technique for forming thin films. An X-ray detection plate which is a detecting unit of an X-ray detector must satisfy functional requirements in that the quantity of a current (charge) that flows when it is not irradiated by an X-ray should be small; the quantity of a current (charge) when irradiated by an X-ray should be great; response at switching between non-irradiated and irradiated states should be high; and fluctuations of charge as a result of repeated use should be small. In particular, an essential requirement for an X-ray image detector is high resolution.
It is an object of the invention to improve the performance of X-ray detection plates used in X-ray detectors as described above, especially X-ray detection plates utilizing amorphous Se and X-ray detectors utilizing such X-ray detection plates. Various structures have been conceived for conventional X-ray detection plates utilizing amorphous Se with a primary intention of increasing the quantity of a current that flows, i.e., sensitivity when irradiated by an X-ray. Although sensitivity is increased, the quantity of a current (charge) that flows when a detection plate is not irradiated with an X-ray, i.e., a dark noise, increases, and a reduction in resolution occurs in the case of a two-dimensional X-ray detector. In practice, no X-ray detection plate has been provided which satisfies all requirements.
A first X-ray detection plate according to the invention is an X-ray detection plate having an insulating substrate, a first electrode film formed on the substrate, a charge transport layer formed on the first electrode film, an X-ray detection layer mainly consisting of amorphous selenium and formed in contact with the charge transport layer and a second electrode film formed on the X-ray detection layer, the charge transport layer being a semi-insulating resistive element having specific resistance between 106 xcexa9cm and 1012 xcexa9cm inclusive, the junction between the charge transport layer and the X-ray detection layer having characteristics of a diode whose cathode is the charge transport layer side and whose anode is the X-ray detection layer side.
A second X-ray detection plate according to the invention is an X-ray detection plate having an insulating substrate, a plurality of charge storage elements formed on the insulating substrate, a plurality of first electrode films formed on the plurality of charge storage elements and electrically connected to the plurality of charge storage elements respectively, a charge transport layer formed on the first electrode films, an X-ray detection layer mainly consisted of amorphous selenium and formed in contact with the charge transport layer and a second electrode film formed on the X-ray detection layer, the X-ray detection plate having means for sequentially reading charge signals stored in the plurality of charge storage elements according to a time series, the charge transport layer being a semi-insulating resistive element having specific resistance between 106 xcexa9cm and 1012 xcexa9cm inclusive.
The second X-ray detection plate may be an X-ray detection plate in which the junction between the charge transport layer and the X-ray detection layer has characteristics of a diode whose cathode is the charge transport layer side and whose anode is the X-ray detection layer side.
The first and/or second X-ray detection plate according to the invention may be an X-ray detection plate in which the thickness of the charge transport layer is between 0.01 xcexcm and 50 xcexcm inclusive and in which the selenium content of the X-ray detection layer is 90 weight % or more.
The first and/or second X-ray detection plate according to the invention may be an X-ray detection plate in which the charge transport layer is a semi-insulating resistive element mainly consisted of diantimony trisulfide (Sb2S3).
The first and/or second X-ray detection plate according to the invention may be an X-ray detection plate in which the second electrode film is a metal film mainly consisted of gold.
The first and/or second X-ray detection plate according to the invention may be an X-ray detection plate in which the first electrode film have transparency.
The first and/or second X-ray detection plate according to the invention may be an X-ray detection plate in which an ITO (indium tin oxide) film is used as the first electrode film having transparency.
The first and/or second X-ray detection plate according to the invention may be an X-ray detection plate in which the substrate have transparency when the first electrode film have transparency.
A first X-ray detector according to the invention comprising an X-ray detection plate, a power supply and an X-ray irradiator, wherein the power supply supplies a voltage to the X-ray detection plate and the X-ray irradiator is caused to emit an X-ray such that the X-ray detection plate is irradiated with the X-ray which has been transmitted by an object to be measured to form an image on the X-ray detection plate, and the X-ray detection plate having an insulating substrate, a first electrode film formed on the substrate, a charge transport layer formed on the first electrode film, an X-ray detection layer mainly consisted of amorphous selenium and formed in contact with the charge transport layer and a second electrode film formed on the X-ray detection layer, the charge transport layer being a semi-insulating resistive element having specific resistance between 106 xcexa9cm and 1012 xcexa9cm inclusive, the junction between the charge transport layer and the X-ray detection layer having characteristics of a diode whose cathode is the charge transport layer side and whose anode is the X-ray detection layer side, the power supply applying a voltage to the second electrode film during the irradiation with the X-ray, the voltage being lower than the voltage applied to the first electrode film.
A second X-ray detector according to the invention is an X-ray detector having an X-ray detection plate, a power supply and an X-ray irradiator, wherein the X-ray irradiator is caused to emit an X-ray while a voltage is applied to the X-ray detection plate by the power supply to form an image on the X-ray detection plate, and the X-ray detection plate having an insulating substrate, a plurality of charge storage elements formed on the insulating substrate, a plurality of first electrode films electrically connected to the plurality of charge storage elements respectively, a charge transport layer formed on the first electrode films, an X-ray detection layer mainly consisted of amorphous selenium and formed in contact with the charge transport layer and a second electrode film formed on the X-ray detection layer, wherein the X-ray detection plate has means for sequentially reading charge signals stored in the plurality of charge storage elements according to a time series and in which the charge transport layer is a semi-insulating resistive element having specific resistance between 106 xcexa9cm and 1012 xcexa9cm inclusive, the power supply applying a voltage to the second electrode film during the irradiation with the X-ray, the voltage being lower than the voltage applied to the first electrode films.
The first and/or second X-ray detector according to the invention may be an X-ray detector having a display device for displaying the image formed on the X-ray detection plate.